Such vertical door structures include two or more glass sheets which, together with a frame embracing the edges of the glass sheets, form a door leaf. The goods are visible through the door, which can be opened to allow a customer to take either a refrigerated or frozen article from the display, as the case may be. The doors to which this invention pertains are vertically disposed doors, usually hinged on a vertical hinge axis.
When the door is closed, the door surface which faces towards the colder storage space will have a lower temperature than the door surfaces which face towards the warmer shop area.
One problem with doors of this kind is that the outer door surface, i.e., the surface of the outermost pane which faces the warm and humid store air, is cooled to a lower temperature than ambient temperature, such that the surface becomes fogged by condensation of moisture contained in the shop air onto that surface. Naturally, such fogging will obscure the view through the door. Another problem is that when the door is opened, moisture in the store air will condense on the inner surface of the door, i.e., the surface of the innermost pane which normally faces towards the colder refrigerator or freezer space.
The first of these problems has been solved, by using a door in the form of a sealed glazing unit, containing two or three panes of glass, wherein the outer pane, on the rear side of the pane, is provided with an electrically conductive coating, which heats the outer pane electrically. The sealed glazing units used in such door structures normally comprise three glass panes.
The use of a sealed glazing structure of high thermal insulating ability, however, results in a high temperature gradient across the door, which means that, in use, the temperature of the inner door surface will be very low. This results in fogging of said surface when the door is opened, e.g., by a customer, and in some cases results in moisture freezing solid on said surface. Subsequent to reclosing the door, this moisture or frost will disappear in time, due to the low dew point prevailing in the refrigerator or freezer space, as the case may be. Because such refrigerator or freezer spaces in vertical cabinets are equipped with forced circulation of internal cooling air, the mist and frost will disappear relatively quickly, as compared to the case of horizontal freezers or refrigerators where normally there is no forced air circulation and instead free convection is utilized. An example of the later type of horizontal chest freezer is found in my U.S. Pat. No. 4,896,785, dated Jan. 30, 1990.
It is known that the time normally taken for such mist or frost to disappear completely from the inner surface of a vertically disposed glass door pane unit is of relatively long duration. The problem resides in the inability of the mist to disappear quickly enough, before the door is again opened by the next customer which results in further fogging of the door surfaces, and so on. These circumstances can, in some cases, result in the build-up of frost or ice on the inner surface of the door. In many instances, the transparency of the door is greatly impaired or lost completely as a result of such fogging and frosting of the door surface, which seriously detracts from the effectiveness of the display.
Since the decisive factor as to whether or not the door is kept free from mist formation on its glass surface is the time lapsed between successive opening of the door, it is important to endeavor to reduce the time taken to dispel the mist formed on the glass surfaces as a result of opening the door.